The invention relates to citraconimide (co)polymers, a process for curing citraconimides with an anionic catalyst, and to articles of manufacture comprising the citraconimide (co)polymers.
Citraconimides and substituted citraconimides are known compounds. The radical copolymerizations of N-substituted citraconimides with styrene and methyl methacrylate are known from the Polymer Journal, Vol. 12, No. 11, pp. 799-807 (1980) and Polymer Journal, Vol. 13, No. 1, pp. 65-74 (1981) wherein azobisisobutyronitrile in a tetrahydrofuran solvent is employed as a polymerization initiator.
World patent application PCT/EP 90/01078 discloses the anionic curing of biscitraconimide (co)polymers using a process similar to that of the present invention. This non-prepublished application however, is limited to biscitraconimides and their copolymers and does not disclose the anionic curing of citraconimides.
Further, an attempt was made in, "Base-Catalyzed Polymerization of Maleimide and Some Derivatives and Related Unsaturated Carbonimides," Kojima, K., et al., Journ. Polym. Sci.: Part A-B, Vol. 4, 1121-1134 (1966), to cure citraconimides with a basic catalyst. In Table VII, however, it is indicated that no polymers or oligomers of the citraconimide were obtained from the reaction at 80.degree. C. over a 24 hour period using sodium-tert-butoxide as the curing catalyst.
In "The Development of Tough Bismaleimide Resins," Stenzenberger, H.D., et al., 31st International SAMPE Symposium, Vol. 31, pp. 920-932 (1986) it is disclosed that bismaleimides are prime candidates for carbon fiber reinforced composites because of their properties. The article also discloses that bismaleimides have been cured in the presence of ionic curing catalysts such as imidazoles and tertiary amines including diazobicyclo-octane (DABCO).
In "Bismaleimide Resins the Properties and Processing of Compimide, BMI Resins," Segal, C.L., et al., 17th Nat. SAMPE Conference 17, pp. 147-160 (1985) formulated bismaleimides are cured with the ionic catalysts DABCO and 2-methyl imidazole.
European Patent Application 0,108,461 published on May 16, 1984, discloses, in example 4, the curing of bismaleimide resins having therein styrene, diallyl phthalate and acrylic acid, in the presence of DABCO.
Japanese patent publication no. 1230612 describes the use of anionic polymerization catalysts for the preparation of a vinyl block copolymer of styrene derivatives and n-substituted maleimides. DABCO is not mentioned in this publication.
Finally, Polymer, Vol. 6, pp. 419-426 (1965) discloses the anionic polymerization of some n-substituted maleimides with butyl lithium.
Generally, the bismaleimide resins require difficult processing conditions, exhibit solvent retention in the prepregs, have a high melting point and high curing temperatures are required for the monomer. In addition, the maleimide polymers are often brittle due to the high cross-link density obtained in the network polymers. The foregoing body of prior art reflects the need for imide resin systems which are easily processable and exhibit improved properties.